"phase change" cooling?

From the various sites I looked at, it seems that this stuff doesn't actually feel like you are putting something cool on, but rather, when you go past a certain temperature, it sucks up the excess heat so you feel more comfortable.

As you can see from the link, there are various items. But the thing I would be most interested in would be something to wear inside my helmet since, whenever you stop for a traffic light or something, the heat builds up pretty quick, especially if you have been pushing hard (and I have lots of traffic lights).

This is a rate equation with respect to time, so you need to work temperatures at various points in time, not just at some arbitrary starting point. Two laws of thermodynamics must be adhered to:

1. heat flows along a gradient, from hotter to cooler

2. total heat does not change, it goes from one place to another, but it is never destroyed

So when you're riding you've got two things to consider. The heat-source is your body and you'll generate a certain amount of heat, BTUs per hour, based upon how much you're exercising. This heat is higher than the outside ambient temperatures so some of it is radiated away. A more significant amount goes into heating up sweat and evaporating it, which absorbs away 540kcal/mole of sweat. A significant factor in radiating and evaporating heat away is surface-area.

As for heat in the helmet, that's a hard one to work with. You've got limited amounts of exposed surface area to radiate and evaporate away heat. I suspect the fastest way to cool your brain would be to have as much surface-area as possible, no helmet at all. we can use that as a 100% control figure. I'm not how to measure how much a helmet blocks at rest at a stoplight. With that figure, we can then compare cooling rates with and without something between your head and the helmet.

The thing is, increasing one cooling method may be counter-productive to the other. using something to faciliate radiating heat away may slow down evaporative cooling and vice-versa... I supposed the best method would be to have something between your head and the helmet that's continually at a lower-temperature than your head AND the outside air... which would require active refridgeration...

Yeah, I've thought of those things, but from what I've read, this stuff does work except I don't know how well the helmet stuff works on a bike because of the reasons you gave.

I find the coolmax headcovering I wear works well for me without causing overheating and it keeps my hair from getting all wet from sweat. Of course, that thin covering still allows the air to do it's job. This other stuff I assume is a little thicker and maybe has its own problems. Sure would like to test it though!

Yeah, that's how sweating works. It evaporates on your skin and the phase-change absorbs heat. How a lot of wicking-fabrics work is that they take the sweat that's trapped between your skin and the jersey that normally just drips down the inside, and wicks it up to the outer surface of your jersey. It's evaporated there on the outside and this cools the outer layer of the jersey. which then sucks up heat away from your skin.

Kinda like how a refridgerator works... I'm looking at the vest. For the phase-change bag of liquid to work, the layer next to your skin has to change from liquid to vapor. This absorbs heat next to your skin. Then this vapor needs to be transmitted to the outer layer of the bag where it's converted back to liquid. This gives off heat which is transmitted through the bag to the outer surface. This heat needs to evaporate something on the outer surface. Maybe sweat that's been pumped from between the skin and inner-layer of the bag to the outside somehow?

Ya know I've given some thought to those for the motorcycle. Gets bloody hot under all that gear. But I have to wonder: Wouldn't a kidney belt with those gel packs you can freeze/microwave be better for less fuss? Huge blood supply in the area so all you do is help it cool/heat as you wish without having a wind blocking cumbersome vest on. The principle holds nicely when I'm wrenching in my un-heated garage wearing one of those Therma-care jobs.